What would you do to zero the sextant if the Index Error is positive?

To zero the sextant when the Index Error is positive, the correct action is to subtract the error. The Index Error refers to the discrepancy between the actual angle being measured and what the sextant indicates. A positive Index Error means that the sextant is reading higher than the true angle. By subtracting this positive error from the sextant's reading, you effectively calibrate it, ensuring that you are accounting for this discrepancy and obtaining accurate measurements.

This calibration is crucial in navigation since precise angles are necessary for determining one's position at sea. The other options, such as adding the error or discarding the error, would lead to incorrect readings and potentially jeopardize navigation efforts. Recording the observation is essential, but it does not adjust the error, thus failing to ensure that the sextant gives an accurate representation of the measured angle. Therefore, subtracting the positive Index Error is the correct way to adjust the sextant for accurate navigation.